The invention is based on a cylinder-piston unit having a central valve, as generically defined hereinafter. Such a unit is already known from U.S. Pat. No. 4,550,567, in which the central valve shaft is embodied as a screw, the cylinder head of which reaches behind the spring plate remote from the piston. Toward the piston, the valve shaft is screwed into a valve body carrying the closing element of the central valve, and the actuating spring engages this valve body. In turn, the valve body is longitudinally guided in a piston in a bore that has a flat valve seat on its bottom.
Stringent demands for the functional reliability of such cylinder-piston units having a central valve are made, particularly if the unit is to be used in vehicle brake systems. However, the known unit has the disadvantage that the screw connection between the valve shaft and the valve body is not secured against loosening. As a result, a change in the spacing between the closing element and the valve seat can occur, which disadvantageously causes a change in the closing travel of this valve. As a result, because of a change in the spacing between the spring plates, a change in the tethering arrangement of the restoring spring also ensues. Moreover, the screw connection allows errors in alignment between the valve shaft and the valve body, which disadvantageously change the closing behavior of the central valve.